Modern optical scanners, such as flatbed scanners, generally employ a moveable sensor bar (e.g., scan head, scan bar), that as part of a scanning operation, moves under a transparent surface where an object (e.g., document, image) has been placed for scanning. However, when the scanner is disabled (e.g., power turned off, power failure), the sensor bar is commonly at an unknown location somewhere along the scanner.
Therefore, upon initialization (e.g., power-up) of the scanner, the location of the sensor bar is unknown. The location of the sensor bar must be determined at a known location so that the scanner can, for example, initiate sensor calibration procedures (e.g., compensate for lighter and darker portions of the scan head), and determine a known location of a top or a bottom of the transparent surface to prepare for a scanning operation.
Contact sensors (e.g., physical location sensors, contact switches, optical location sensors) are sometimes used in scanners to detect when a sensor bar has reached a known (or predetermined) location. Such a known location may be commonly referred to as a “home” location or a location that is relative to a “home” location. However, employing one or more contact sensors adds manufacturing cost to a scanner. Additionally, failure of a contact sensor would prevent a scanner from finding a known location of the sensor bar. Moreover, the sensor bar could be moved past a defective contact sensor to an end of an internal chamber of the scanner, where the motor would continue to run even though the sensor bar cannot be moved any further. This could potentially damage the motor used to drive the sensor bar.